Apart from "that that", are there other words that ever make sense repeated?

You mean the Western lowland gorilla (Gorilla gorilla gorilla)? – Gorilla gorilla subspecies are so confusing.

And, in keeping with the buffalo theme, the magnificent beast of the American plains is Bison bison bison.

Not to mention the water buffalo Bubalis bubalis bubalis.

Try as I might, I cannot parse this, though it sounds very cool!

Ha ha.

the second “which” should technically be a “who”

I don’t mind icing cakes, but icing icing is ridiculous.

(There are probably a lot more like this)

A horny stag does does.
I have worn worn shoes.
In a storm, you might see a shed shed its roof.

  • but not a stable, if it were a stable stable.
    A pile of hampers hampers his escape from the laundry room.
    The judgement was not harsh; just just.

Random trivia: such genus-species pairs are known as tautonyms.

Wenn hinter Fliegen Fliegen fliegen, fliegen Fliegen Fliegen nach.

..

Which typically stands in water higher than its tailbone, and is a martyr to flatulence - hence its name.

I just happened to write this sentence and thought of this thread, “I said that I would be in in the morning.”

And when Christopher Robin said it to an Egyptian pharaoh, he said, “Tut-tut, Tut, it looks like rain.”

There’s no there there.

Aye aye, captain.

I’m surprised no one’s mentioned great yet, and relative-naming conventions in general.

My great great grandfather was a great great great grandfather. He was my father’s father’s father’s father.

Yeah, yeah.

and, if I may quote the Fab Four,

Yeah, yeah, yeah!

He got a boo boo.